Long wavelength vertical cavity surface emitting lasers (VCSELs) are attractive for long reach optical communication applications. Fabrication of long wavelength VCSELs incorporating well-developed AlAs/GaAs distributed Bragg reflectors (DBRs) is achieved by growing highly strained InGaAsN active regions on GaAs. Long wavelength VCSEL structures include structures where InGaAsN active regions are sandwiched between a first and second cladding region of AlGaAs/GaAs or InGaP/GaAs and top and bottom AlGaAs/GaAs DBR mirror layers.
Research by Kawaguchi et al. in Electronics Letters, 36, 2000, 1776 indicates that the material quality of metal-organic chemical vapor deposition (MOCVD) InGaAsN is severely degraded if the InGaAsN quantum well active layer is grown directly on the AlGaAs/GaAs DBR and lower cladding layers. To achieve acceptable material quality for the quantum well active layer, two separate reactors are used to grow the wafers for 1.3 xcexcm wavelength VCSELs with InGaAsN quantum well active layers. A first reactor is used to grow the AlGaAs/GaAs DBR and lower cladding layers. Subsequently, the wafer is transferred to a second reactor for the growth of InGaAsN quantum well active layers, the top cladding layer and the top DBR mirror layers. These long wavelength InGaAsN VCSELs have xe2x80x9csymmetricxe2x80x9d structures where both the top and bottom cladding layers have the same composition. Sato et al. in Electronics Letters, 36, 2000, 2018 disclose an xe2x80x9casymmetricxe2x80x9d VCSEL structure grown in a two reactor MOCVD process where a GaInP layer functions as an etch stop.
The use of an InGaAsN quantum well active layer allows VCSEL operation in the important 1300 nm or longer wavelength regime which is of interest for telecommunications and Internet infrastructure applications. In accordance with the invention, an asymmetric InGaAsN VCSEL structure may be made which allows all growth steps to be performed in the same metal-organic chemical vapor deposition (MOCVD) reactor.
In the asymmetric VCSEL structure, the first AlGaAs/GaAs DBR mirror layer is followed by growth of a sufficiently thick nitrogen or nitrogen and phosphorus containing layer such as GaAsN, InGaAsPN, GaAsPN, GaAsN, AlGaAsN, InGaPN, or similar compositions to improve growth of the InGaAsN quantum well active layer by serving to getter Al while not interrupting the MOCVD growth process. The top cladding layer may be AlGaAs to provide for higher band offset resulting in better electron confinement than is provided by a nitrogen or phosphorus containing cladding layer. However, AlGaAs requires a more complicated growth structure and typically GaAs is used for the top cladding layer. Instead of using a C-doped GaAs contact layer, a reverse-biased tunnel junction can be used to form the p-contact to reduce resistance and optical losses.
Using an asymmetric InGaAsN VCSEL structure results in the InGaAsN quantum well active layer having a quality that is comparable to that achieved by the conventional two reactor MOCVD process while providing good laser performance along with lower production costs by using a single reactor MOCVD process.